1. Field of the Invention
The present invention relates to an apparatus for measuring circuit constants of an induction motor using a vector control and a method therefor. The present invention particularly relates to a speed variable apparatus for the induction motor having a current control system (ACR, generally abbreviated for an automatic current regulator) and having a function of measuring motor constants. The motor constants are described in an established equivalent circuit of the induction motor.
2. Description of the Background Art
In a vector control system of a speed variable apparatus for an induction machine such as an induction motor, it becomes necessary to know an actual time constant of a rotor (secondary circuit) in order to calculate a slip frequency of the motor since the vector control is required to control currents and slip to control the slip frequency. In addition, since a secondary circuit resistance R.sub.2 tends to vary according to the rotor temperature (ambient temperature inclusive), it becomes necessary to know all of the circuit constants of the motor to compensate for the variation in the secondary circuit resistance R.sub.2.
One of previously proposed motor circuit constant measuring methods includes a standard specified by JEC-37-.sub.1979 (Standard of a Japanese Electrotechnical Committee INDUCTION MACHINES). The JEC 87 describes measurements of a primary (stator) resistance using a DC potential difference drop method, a leakage inductance and secondary circuit (rotor) resistance using a locked rotor test, and an exciting inductance using a no-load test.
It is necessary to provide means for fixing an output axle of the motor thereon in the locked rotor test and often necessary to provide a large sized instrument for its measurement of the leakage inductance.
In addition, since, for its measurement condition, a measured voltage is relatively low so that a measurement with a higher frequency than frequency components generated in the secondary circuit of the motor during an actual driving of the motor is made so as to provide a desired voltage accuracy.
In the locked rotor test of the induction motor formed of a special shape such as a double squirrel-cage type as a secondary conductor, an error occurs due to the influence of a skin effect between values of the leakage inductance measured at a higher frequency than that during the driving of the motor and of that during the actual driving situation.
Hence, in the cases where the measured constants derived through the locked rotor test under the JEC 37 are applied to the vector control system for the induction motor and/or where the highly functional control such as the temperature variation compensation for the secondary resistance in the vector control system is carried out, an error occurs in an output torque of the motor and it becomes difficult to control the motor with a high accuracy.
As described above, since the above-described method of measuring the motor circuit constants under the specification of JEC 37 may accurately not derive the motor constants under actual motor driving situations, in the vector control system, load torque tests are actually carried out so that an operator is manually needed to adjust the motor constants so that results of testing characteristics and transient characteristics accord with ideal characteristics.